Due to rapid development in ink jet recording technology, it has become possible to form a colored and high quality image on a recording medium such as paper and film by printers using ink jet recording system. Such printers using ink jet recording system vary from small size printers for home use to wide-format printers used by printing companies and so forth. Since printing is basically performed on a one sheet-by-one sheet basis, these printers were mainly used at printing sites where a small number of copies was required.
In recent years, because of further development of the technology, application of ink jet recording system to commercial printing (hereinafter referred to as “ink jet printing”) has been started. In the commercial printing field, the number of copies to be printed is large and in the light of the balance between productivity and printing cost, printing speed is valued. Printing speed suitable for ink jet printing is achieved by a printing machine comprising a line scan head to which heads for ejecting ink are fixed such that they cover the entire cross direction intersecting at right angles with the machine direction (hereinafter referred to as “ink jet printing machine”) (see, for example, Patent Document 1). Recently, rotary-type ink jet printing machines have also been developed. And rotary-type ink jet printing machines with a printing speed of 15 m/min or more, those with a higher speed of 60 m/min or more, and those with a speed exceeding 120 m/min have been developed.
Since ink jet printing machines can deal with variable information, they are applied particularly to on-demand printing. It is preferred that fixed information be printed by an offset printing machine, and variable information be printed by an ink jet printing machine in commercial printing.
PPC paper or non-coated paper has conventionally been used as paper for ink jet printing machine. There is a demand, however, for coated printing paper which has a texture similar to that of general-purpose coated printing paper such as CWF matt coated paper and CWF gloss coated paper, and can be used for ink jet printing machines in order to meet the increasing demand for high definition and high quality commercial printing.
In view of weather resistance, ink jet printing machines which use pigment ink as ink jet ink are increasing. As problems of pigment ink, poor dot diffusion and poor abrasion resistance can be mentioned. “Poor dot diffusion” refers to a phenomenon of insufficient ink spread in a planar direction during the process where ink jet ink collides with and is absorbed by coated printing paper. As a result of poor dot diffusion, streaky areas, i.e., white lines where overlapping of inks is insufficient, appear on the printed image. It is therefore necessary to inhibit poor dot diffusion. “Poor abrasion resistance” refers to a phenomenon where ink comes off from a printed portion of a coated printing paper by being rubbed with something. Due to such poor abrasion resistance, smudges occur on printed image. It is therefore necessary to improve abrasion resistance property.
As coated printing paper for ink jet printing machines, there are following recording media: those in which the base paper absorbs ink 1.5 times as much as the coating layer does (see, for example, Patent Document 2); those having a specific degree of water content and stiffness in CD/MD direction (see, for example, Patent Document 3); those comprising a base paper, a binder layer and further a second layer containing kaolin and polyvinyl alcohol, and having a specific water absorption degree (see, for example, Patent Document 4). These papers, however, have no sufficient ink fixing property nor ink absorption property for ink jet printing.
As recording papers where the contact angle and droplet absorption time are specified, there are following papers exclusive use for ink jet printers: those having an absorbed amount of 0.15 μl or more and a contact angle of 50° or more of 2 μl of pure water, after 5 seconds from dropping thereof (see, for example, Patent Document 5); those having a contact angle of 40° to 80°, both inclusive, of water, after 0.04 second (see, for example, Patent Document 6); those having a contact angle of 10° to 30°, both inclusive, of an deionized water after 0.5 second (see, for example, Patent Document 7); those having an absorption time of 60 seconds or less of 5 μl of a liquid having a surface tension of 40 mN/m and a contact angle of 50° to 80°, both inclusive, of the liquid after 0.1 second (see, for example, Patent Document 8). These exclusive papers for ink jet printers where the contact angle and liquid absorption time are specified, however, are for printers which perform printing on a one sheet-by-one sheet basis, and thus they fail to have ink jet printability.    [Patent Document 1] Japanese Laid-open Patent [Kokai] Publication No. 2009-23292    [Patent Document 2] Japanese Laid-open Patent [Kokai] Publication No. 2007-118289    [Patent Document 3] Japanese Laid-open Patent [Kokai] Publication No. 2007-83681    [Patent Document 4] Japanese Laid-open Patent [Kokai] Publication No. 2009-125947    [Patent Document 5] Japanese Laid-open Patent [Kokai] Publication No. 2007-185780    [Patent Document 6] Japanese Laid-open Patent [Kokai] Publication No. 2005-88482    [Patent Document 7] Japanese Laid-open Patent [Kokai] Publication No. 2005-153221    [Patent Document 8] Japanese Laid-open Patent [Kokai] Publication No. 2002-347328